A person has been fasting for 24 hours and liver glycogen is nearly depleted. Which process is now the primary source of blood glucose?
AGlycogenolysis from muscle glycogen released into the bloodstream
BGluconeogenesis from lactate, glycerol, and amino acids in the liver
CIncreased intestinal absorption of dietary glucose from slow digestion
DDirect conversion of ketone bodies back into glucose by the liver
After 12–18 hours of fasting, liver glycogen is exhausted. Muscle glycogen cannot contribute to blood glucose because muscle lacks glucose-6-phosphatase — it can only fuel the muscle itself. Ketone bodies cannot be converted back to glucose (the reaction is irreversible). The liver must therefore synthesize new glucose from non-carbohydrate precursors: lactate (from anaerobic glycolysis), glycerol (from lipolysis of triglycerides), and glucogenic amino acids from muscle protein breakdown. This is gluconeogenesis, the dominant glucose source during prolonged fasting.
Question 2 Multiple Choice
After several days of fasting, the rate of muscle protein breakdown decreases significantly. What causes this reduction?
AThe body has fully depleted amino acid stores, so there is nothing left to break down
BRising insulin levels signal muscles to halt proteolysis
CThe brain adapts to use ketone bodies for most of its energy, reducing the demand for gluconeogenesis and thus for amino acid substrates
DAMPK directly inhibits the proteasome once fatty acid levels reach a threshold
Early in fasting, the brain's strict requirement for glucose forces the liver to run gluconeogenesis at high rates, which requires amino acids as substrates, which requires muscle breakdown. But after several days, the brain adapts to derive 60–70% of its energy from ketone bodies rather than glucose. This dramatically reduces the glucose requirement, and thus the demand for gluconeogenic amino acids, and thus the rate of muscle protein catabolism. The key insight is that ketogenesis is not a sign of metabolic failure — it is a protective adaptation that preserves lean body mass.
Question 3 True / False
During an extended fast, the brain relies exclusively on glucose for energy throughout the entire fasting period.
TTrue
FFalse
Answer: False
This is a persistent misconception. The brain has an absolute requirement for glucose that cannot be circumvented in the short term, but after several days of fasting it adapts to use ketone bodies (acetoacetate and β-hydroxybutyrate) for up to 60–70% of its energy. This ketone adaptation is critical: it allows the body to slow the breakdown of muscle protein, which would otherwise be unsustainable. Glucose remains essential for the remaining ~30–40% of brain energy and for red blood cells (which have no mitochondria and cannot use ketones).
Question 4 True / False
In the fasted state, fatty acids become the primary fuel for muscles and other tissues, which spares glucose for organs that cannot use alternatives.
TTrue
FFalse
Answer: True
This glucose-sparing shift is a core feature of the fasted state. As fatty acid oxidation supplies most of the energy for muscle, heart, and other tissues, the limited gluconeogenic output from the liver is preserved for the brain and red blood cells. The body essentially redirects its fuel hierarchy: glucose becomes scarce and reserved for essential consumers, while fatty acids become the abundant general-purpose fuel.
Question 5 Short Answer
What role does AMPK play in the fasted state, and how does its function complement the hormonal signal from glucagon?
Think about your answer, then reveal below.
Model answer: AMPK (AMP-activated protein kinase) is a cellular fuel gauge that activates when ATP falls and AMP accumulates. It promotes catabolic pathways (fatty acid oxidation, autophagy) and inhibits anabolic ones (fatty acid synthesis, protein synthesis). Glucagon signals the whole organism to mobilize fuel — through the bloodstream, affecting liver and adipose tissue. AMPK operates within each cell to shift its own metabolism to match: a cell experiencing energy stress activates AMPK independently of circulating hormones. Together, they create a two-layer coordination: hormonal signaling sets the systemic context, and AMPK ensures each cell's internal machinery aligns with it.
The distinction between hormonal and intracellular energy sensing is important: AMPK can respond to local energy deficits even when circulating hormone levels haven't changed, and it can maintain the fasted metabolic state in peripheral tissues during periods when blood glucagon levels fluctuate. This redundancy makes the fasting response robust to noise in hormonal signaling.